The study of absorption
spectra by means of passing electromagnetic radiation through an atomic medium
that is selectively absorbing; this produces pure electronic transitions free
from vibrational and rotational transitions

Figure1. Elements detectable by
atomic absorption are highlighted in pink in this periodic table

Atomic absorption
methods measure the amount of energy (in the form of photons of light, and thus
a change in the wavelength) absorbed by the sample. Specifically, a detector
measures the wavelengths of light transmitted by the sample (the
"after" wavelengths), and compares them to the wavelengths, which
originally passed through the sample (the "before" wavelengths). A
signal processor then integrates the changes in wavelength, which appear in the
readout as peaks of energy absorption at discrete wavelengths (see schematic of an atomic-absorption experiment).

Any atom has its own distinct pattern of wavelengths at which
it will absorb energy, due to the unique configuration of electrons in its
outer shell. This allows for the qualitative analysis of a pure sample.

In order to tell how much
of a known element is present in a sample, one must first establish a basis for
comparison using known quantities. It can be done producing a calibration curve. For this process, a known
wavelength is selected, and the detector will measure only the energy emitted
at that wavelength. However, as the concentration of the target atom in the
sample increases, absorption will also increase proportionally. Thus, one runs
a series of known concentrations of some compound, and records the
corresponding degree of absorbance, which is an inverse percentage of light
transmitted. A straight line can then be drawn between all of the known points.
From this line, one can then extrapolate the concentration of the substance
under investigation from its absorbance. The use of special light sources and
specific wavelength selection allows the quantitative determination of
individual components of a multielement mixture.

The phenomenon of atomic
absorption (AA) was first observed in 1802 with the discovery of the Fraunhofer
lines in the sun's spectrum. It was not until 1953 that Australian physicist Sir Alan Walsh demonstrated that atomic absorption could
be used as a quantitative analtical tool. Atomic absorption analysis involves
measuring the absorption of light by vaporized ground state atoms and relating
the absorption to concentration. The incident light beam is attenuated by
atomic vapor absorption according to Beer's law.

The process of atomic
absorption spectroscopy (AAS) involves two steps:

1.Atomization
of the sample

2.The
absorption of radiation from a light source by the free atoms

3.

The sample, either a
liquid or a solid, is atomized in either a flame or a graphite furnace. Upon
the absorption of ultraviolet or visible light, the free atoms undergo
electronic transitions from the ground state to excited electronic states.

To obtain the
best results in AA, the instrumental and chemical parameters of the system must
be geared toward the production of neutral ground state atoms of the element of
interest. A common method is to introduce a liquid sample into a flame. Upon
introduction, the sample solution is dispersed into a fine spray, the spray is
then desolvated into salt particles in the flame and the particles are
subsequently vaporized into neutral atoms, ionic species and molecular species.
All of these conversion processes occur in geometrically definable regions in
the flame. It is therefore important to set the instrument parameters such that
the light from the source (typically a hollow-cathode lamp)
is directed through the region of the flame that contains the maximum number of
neutral atoms. The light produced by the hollow-cathode lamp is emitted from excited
atoms of the same element which is to be determined. Therefore the radiant
energy corresponds directly to the wavelength which is absorbable by the
atomized sample. This method provides both sensitivity and selectivity since
other elements in the sample will not generally absorb the chosen wavelength
and thus, will not interfere with the measurement. To reduce background
interference, the wavelength of interest is isolated by a monochromator placed
between the sample and the detector.

In atomic absorption
(see schematic of an atomic-absorption experiment), there
are two methods of adding thermal energy to a sample. A graphite furnace AAS
uses a graphite tube with a strong electric current to heat the sample. In
flame AAS (see photo above), we aspirate a sample into a flame using a nebulizer. The flame is lined up in a beam of light of
the appropriate wavelength. The flame (thermal energy) causes the atom to
undergo a transition from the ground state to the first excited state. When the
atoms make their transition, they absorb some of the light from the beam. The
moreconcentrated the solution, the more light energy is absorbed!

The light beam is
generated by lamp that is specific for a target metal. The lamp must be
perfectly aligned so the beam crosses the hottest part of the flame. The light
passed through the flame is received by the monochromator, which is set to accept
and transmit radiation at the specified wavelength andtravels into the detector. The detector measures the intensity of the
beam of light. When some of the light is absorbed by metal, the beam's
intensity is reduced. The detector records that reduction as absorption. That
absorption is shown on output device by the data system.

We can find the
concentrations of metals in a sample running a series of calibration standards
through the instrument. The instrument will record the absorption generated by
a given concentration. By plotting the absorption versus the concentrations of
the standards, a calibration curve can be plotted. We
can then look at the absorption for a sample solution and use the calibration
curves to determine the concentration in that

Atomic absorption
spectrometry is a fairly universal analytical method for determination of
metallic elements when present in both trace and major concentrations. The EPA
employs this technique for determining the metal concentration in samples from
a variety of matrices.

A) Sample preparation

Depending on the
information required, total recoverable metals, dissolved metals, suspended metals,
and total metals could be obtained from a certain environmental matrix. Table 1
lists the EPA method number for sample processing in terms of the environmental
matrices and information required. For more detail information, readers could
refer to EPA document SW-846 "Test methods for evaluating solid wastes".

Appropriate acid
digestion is employed in these methods. Hydrochloric acid digestion is not suitable
for samples, which will be analyzed by graphite furnace atomic absorption
spectroscopy because it can cause interferences during furnace atomization.

The idealized
calibration or standard curve is stated by Beer's law
that the absorbance of an absorbing analyte is proportional to its
concentration.

Unfortunately,
deviations from linearity usually occur, especially as the concentration of
metallic analytes increases due to various reasons, such as unabsorbed
radiation, stray light, or disproportionate decomposition of molecules at high
concentrations. Figure 3 shows an idealized and deviation of response curve.
The curvature could be minimized, although it is impossible to be avoided
completely. It is desirable to work in the linearity response range. The rule
of thumb is that a minimum of five standards and a blank should be prepared in
order to have sufficient information to fit the standard curve appropriately.
Manufacturers should be consulted if a manual curvature correction function is
available for a specific instrument.

Figure 3. Idealized/deviation
response curve

If the sample
concentration is too high to permit accurate analysis in linearity response
range, there are three alternatives that may help bring the absorbance into the
optimum working range:

1) sample
dilution

2)
using an alternative wavelength having a lower absorptivity

3)
reducing the path length by rotating the burner hand.

C) EPA method for metal
analysis

Flame atomic absorption
methods are referred to as direct aspiration determinations. They are normally
completed as single element analyses and are relatively free of interelement
spectral interferences. For some elements, the temperature or type of flame
used is critical. If flame and analytical conditions are not properly used,
chemical and ionization interferences can occur.

Graphite furnace atomic
absorption spectrometry replaces the flame with an electrically heated graphite
furnace. The major advantage of this technique is that the detection limit can
be extremely low. It is applicable for relatively clean samples, however,
interferences could be a real problem. It is important for the analyst to
establish a set of analytical protocol which is appropriate for the sample to
be analyzed and for the information required. Table 2 lists the available
method for different metal analysis provided in EPA manual SW-846.

Table 2. EPA methods for determination of metals by direct aspiration

Analyte

Method number

Analyte

Method number

Analyte

Method number

aluminum

7020

antimony

7040

barium

7080A

beryllium

7090

cadmium

7130

calcium

7140

chromium

7190

cobalt

7200

copper

7210

iron

7380

lead

7420

lithium

7430

magnesium

7450

manganese

7460

molybdenum

7480

nickel

7520

osmium

7550

potassium

7610

silver

7760A

sodium

7770

strontium

7780

thallium

7840

tin

7870

vanadium

7910

zinc

7951

D) Interferences

Since the concentration
of the analyte element is considered to be proportional to the ground state atom
population in the flame, any factor that affects the ground state population of
the analyte element can be classified as interference. Factors that may affect
the ability of the instrument to read this parameter can also be classified as
interference. The following are the most common interferences:

A) Spectral interferences
are due to radiation overlapping that of the light source. The interference
radiation may be an emission line of another element or compound, or general
background radiation from the flame, solvent, or analytical sample. This
usually occurs when using organic solvents, but can also happen when
determining sodium with magnesium present, iron with copper or iron with
nickel.

B) Formation of compounds
that do not dissociate in the flame. The most common example is the formation
of calcium and strontium phosphates.

C) Ionization of the
analyte reduces the signal. This is commonly happens to barium, calcium,
strontium, sodium and potassium.

D) Matrix interferences due
to differences between surface tension and viscosity of test solutions and
standards.

E) Broadening of a
spectral line, which can occur due to a number of factors. The most common line
width broadening effects are:

1. Doppler effect

This effect arises because atoms will have different
components of velocity along the line of observation.

2. Lorentz effect

This effect occurs as a result of the concentration of
foreign atoms present in the environment of the emitting or absorbing atoms.
The magnitude of the broadening varies with the pressure of the foreign gases
and their physical properties.

3. Quenching effect

In a low-pressure spectral source, quenching collision can
occur in flames as the result of the presence of foreign gas molecules with
vibration levels very close to the excited state of the resonance line.

4. Self absorption or self-reversal effect

The atoms of the same kind as that emitting radiation will
absorb maximum radiation at the center of the line than at the wings, resulting
in the change of shape of the line as well as its intensity. This effect
becomes serious if the vapor, which is absorbing radiation is considerably
cooler than that which is emitting radiation.